Classical central extension for asymptotic symmetries at null infinity in three spacetime dimensions

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The symmetry algebra of asymptotically flat spacetimes at null infinity in three dimensions is the semi-direct sum of the infinitesimal diffeomorphisms on the circle with an Abelian ideal of supertranslations. The associated charge algebra is shown to admit a non-trivial classical central extension of Virasoro type closely related to that of the anti-de Sitter case.